No point buying it again this time, but seriously for future seasons consider buying it direct from TT:

1. You get the DVD and extras (which I'm really gutted I missed out on in previous seasons)

2. Occasionally a Steam release can get delayed, you might say "oh sure I can wait an extra day" - but for ToMI Part 5 it was pretty much the lamest thing ever, since it had been delayed quite a bit already and I'd waited up til 4am for the release only to find out I had to wait til the next evening (UK time).

3. TT get extra money? - Not sure how true this is (after overheads of DVDs, downloads, etc.) but Valve certainly do take a cut from Steam sales.

4. Private preorder forum - again not a major one, but there have been some nice teasers and Q&A threads from the creators, which make for some entertaining reading in between episodes.